Level measurement devices of this type are e.g. used in in industrial applications e.g. for measuring a filling level of a product in a container.
Microwave level measurement devices transmit microwaves toward the surface of the product and receive echo signals of the transmitted microwaves. Based on the echo signals received an echo function representing an echo amplitude as a function of a distance or time is determined during each measurement. Based on the echo function a transit time required for a microwave signal to travel from the device to the surface of the product and of its echo to return to the device is determined. The distance between the device and the surface is then determined based on the transit time.
Various microwave level measuring techniques are known in the art which permit short distances to be measured by means of reflected waves. The most frequently used systems are pulse radar systems and frequency-modulated continuous wave radar systems (FMCW-Radar). Pulse radar level measurement devices periodically send short microwave pulses. The transit time between the transmission of the microwave pulse and the reception of its echo is measured and the level is determined based on the transit time. FMCW radar level measurement devices transmit a continuous microwave signal, which is periodically linearly frequency modulated. The frequency of the received echo signal differs from the frequency of the transmitted signal by an amount, which depends on the transit time between the emission and the reception of the corresponding echo signal.
The microwave signals are transmitted and received via antennas. To this extent, the devices can either comprises a transmitting antenna and a separate receiving antenna or they can comprise a single antenna for sending and receiving microwave signals. Different types of antennas are known in the art, e.g. horn antennas or rod antennas.
German application DE 10 2013 11 42 40 A1 describes a level measurement device for measuring a level of a product, in particular a product contained in a container, comprising:                mounting means for mounting said measurement device at a measurement site,        an antenna comprising a dielectric antenna element for transmitting microwave signals towards the product and/or for receiving echo signals resulting from reflections of the transmitted microwave signals, and        measurement electronics for determining the level of the product based on a transit time needed for the microwave signals to travel to a surface of the product and of their echo signals to return to the device.        
The antenna of this device comprises an essentially ball shaped antenna element for transmission and reception of microwave signals. The antenna is connected to the measurement electronics via a wave guide and the antenna element is mounted onto an end section of the wave guide. To this extent, an extension is foreseen on the antenna element and the extension is secured inside the end section of the wave guide facing the product.
In many applications antennas of measurement devices will be exposed to gases, in particular to air, which may contain particles, in particular dust. In case particles deposit on the antenna, this will alter the transmission properties of the antenna. The extent to which the transmission properties are affected by deposits depends on the thickness and the dielectric properties of the deposits. Deposits affecting the transmission properties of the antenna will in turn affect the measurement properties of the device and thus lead to a reduced measurement accuracy.
In order to overcome this problem measurement devices can be equipped with a rinsing system for removing deposits. A rinsing system for rinsing an inside surface of a horn antenna is for example described in International Application WO2006/063930 A1. This system comprises a rinsing device, which is designed to send a pressurized rinsing medium through a nozzle across the surface of the antenna. The rinsing medium is for example a pressurized gas, e.g. compressed air, or a rinsing liquid.
Rinsing systems require the installation of the rinsing device as well as a reservoir for the rising medium, which will need to be refilled regularly. Since the system is operated under pressure, measures need to be taken to prevent leakages. In consequence installation and maintenance costs of rinsing systems are comparatively high.
With respect to horn antennas, it is known in the art, that the installation and maintenance costs involved in the application of the pressurized rinsing systems can be avoided, if a vibratory cleaning mechanism is used instead of a rinsing system, which causes the horn to vibrate when activated. A vibratory cleaning mechanism for cleaning a horn antenna is described in U.S. Pat. No. 6,917,326 B1. This cleaning mechanism comprises a piezoelectric transducer comprising two piezoelectric rings. The rings are clamped between two sections of a hollow outer conductor of a wave guide connected to the horn. When activated, the transducer generates acoustic waves which propagate in a direction essentially parallel to ta longitudinal axis of wave guide and horn through the horn antenna causing the horn to vibrate. Due to the vibration deposits will be at least partially dislodged or removed from the inner surface of the horn antenna. It is described to tune the frequency of the vibration to the natural resonance frequency of the horn or to adjust or tune frequency and/or amplitude of the vibration to allow optimum performance for the material to be removed from the antenna.
Just like the horns of horn antennas the dielectric antenna elements of antennas comprising dielectric antenna elements for transmitting and/or receiving microwave signals may be exposed to gases containing particles, which may deposit on the dielectric antenna element. Deposits on the antenna element will alter the transmission properties of the antenna. Thus there is a need in industry to protect antennas comprising dielectric antenna elements for transmitting and/or receiving microwave signals from deposits impairing transmission and/or reception of the signals.
It is an object of the invention to provide a level measurement device comprising an antenna comprising a dielectric antenna element for transmitting and/or receiving microwave signals, allowing to prevent deposits to build up, which may eventually impair transmission and/or reception of the signals.